The present invention relates to electronic circuits associated with leads and leadwires disposed within a living body. More particularly, the present invention relates to a transient voltage/surge current protection system wherein a transient voltage suppressor is electrically connected in parallel with an electronic circuit in order to protect the circuit from a surge current which may momentarily develop as a result of, for example, the use of an automatic external defibrillator during a cardiac emergency.
Implanted leads or implanted leadwires are associated with a variety of active implantable medical devices (AIMDs), including cardiac pacemakers, implantable cardioverter defibrillators (ICDs), neurostimulators including deep brain stimulators, spinal cord stimulators and other types of pain control stimulators, and the like. Implanted leadwires may also be associated with probes or catheters which are temporarily inserted into the body. Probes and catheters are used for a variety of applications, including mapping of cardiac signals, cancer ablation and the like. In general, implanted leads or leadwires associated with AIMDs or elongated leads associated with probes or catheters have both a proximal end, external of the AIMD, and a distal therapy delivery or sensing end.
Automatic external defibrillators (AEDs) are now very common and appear in many public places, including government buildings, airports, airplanes, etc. AEDs are generally used by trained personnel who will attend to incidents, however many are public access units which can be found in places including corporate and government offices, shopping centers, airports, restaurants, casinos, hotels, sports stadiums, schools and universities, community centers, fitness centers and health clubs.
An increasing number of patients with AIMDs are undergoing external defibrillation during cardiac emergencies. There have been reports of damage to AIMDs due to use of AEDs during such emergencies. Typically, AIMDs include internal circuit protection devices to protect against these external voltage surges. Defibrillation is the definitive treatment for life-threatening cardiac arrhythmias, ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a defibrillator device. The external defibrillator or AED produces a high energy which depolarizes a critical mass of the heart muscle, terminates the arrhythmia, and allows normal sinus rhythm to be re-established by the body's natural pacemaker in the sinoatrial of the heart. Defibrillators can be external, transvenous or implanted depending on the type of device used. External units, known as automatic external defibrillators (AEDs), automate the diagnosis of treatable rhythms so that lay responders or bystanders are able to use them successfully with little or, in some cases, no training.
For an adult, the nominal delivered pulse from an AED is 150 joules. For an infant or child, the nominal energy delivery is 50 joules. Studies have shown that a biphasic waveform of 115 joules is equivalent to a monophasic wave of about 200 joules. Because of the decreased energy needed, most defibrillators now use biphasic waveforms. The lower energy can result in both longer battery life and a shorter time to full charge for the AED. An AED can supply as much as 2000 volts from its high-energy storage capacitors with an 80 maximum peak ampere for a 25-ohm impedance patient. A 50-ohm patient would receive 40 maximum peak amperes (reference: Association for the Advancement of Medical Instrumentation Standards). The surge currents induced in an implanted leadwire due to the transient voltage introduced through use of an AED could be 2 to 6 amps or even higher.
In order to make AIMDs compatible with medical diagnostic procedures such as magnetic resonance imaging (MRI), a number of lead-based electronic components are being developed. These lead-based components can be bandstop filters, electronic filters, micro-electrical mechanical switches (MEMS), multiplexers, and other types of active electronic filters or switches. See, for example, U.S. Pat. No. 7,363,090 the contents of which are incorporated herein by reference. See also U.S. patent Ser. Nos. 11/558,349, 11/743,000, 11/860,402, 11/930,742, 11/838,035, 11/943,883, 11/943,854, 12/8,489,921, and 61/016,364 the contents all of which are incorporated herein by reference. During external defibrillation from an AED, high currents can be picked up by implanted leads or leadwires. This depends on electrode placement, the physical characteristics of the patient, and also the location of implanted leads or leadwires.
A transient voltage or high surge current need not always come from an external source like an AED. In fact, an implantable cardioverter defibrillator (ICD) senses abnormal cardiac activity, such as dangerous ventricular arrhythmias. When a dangerous ventricular arrhythmia is detected, the ICD delivers a high voltage shock through leads whose electrodes are in intimate contact or associated with the heart. If the ICD leadwire system has any electronic circuits disposed in its leads, the high voltage shock must pass through said electronic circuit. For example, if a bandstop filter, such as one described in U.S. Pat. No. 7,363,090, which shows inductor-capacitor electronic circuits in series with implanted leads, is placed in the high voltage shock delivery electrode circuit of an ICD, then the ICD pulse would have to pass through the bandstop filter. Since the ICD pulse is of a low frequency, the bulk of the pulse would pass through the inductor component of the bandstop filter. However, it is really not practical or feasible to make inductors with large enough wires to handle such high currents as an ICD pulse.
ICD pulses may be monophasic or biphasic. Therefore, it is important to protect circuit components placed in implanted leads from both positive and negative polarity voltage which could result in current surges in either direction in the lead. As for the case with an AED, protection of all kinds of lead based electronic circuits is important. This includes not only bandstop filters, but all types of frequency selective impeding or diverting circuits, MEMS switches, electronic switches, multiplexing switches and the like. Such protection is also needed for a wide range of lead based sensors, including oxygen sensors, pressure sensors, general blood gas sensors, artificial valve lead transducers and the like.
Accordingly, there is a need to provide circuit protection devices for electrical or electronic circuits that are associated with implanted or implantable leads, leadwires and the like. Such circuit protection devices must be of such dimension and construction to lend themselves for use with implantable leads, leadwires and the like, and must not interfere with the normal therapy delivery or sensing functions of the implanted leads or leadwires. Moreover, such circuit protection devices must allow for normal operation of the electrical or electronic circuits disposed in series with the implanted leads or leadwires, and preferably function to divert a surge current around the electrical or electronic circuits in order to bypass the same during a cardiac emergency, for example, a high voltage pulse introduced into the major leadwire system through the use of an AED or an ICD. The present invention fulfills these needs and provides other related advantages.